Sugar production from sugar beet is a continuous process which is very energy intensive and requires large amounts of water. Methods of extracting sugar from natural sugar sources such as sugar beet, sugar cane, generally involve the slicing of the plant material and “diffusing” the sliced material with hot water. The resulting sugar solution is combined with a juice resulting from pressing the exhausted plant material to form the raw juice or sugar liquor. This raw juice contains many organic and inorganic non-sugar impurities including plant derived substances, including both dissolved and undissolved solids, other than sucrose. Before it can be used for sugar production, these impurities must be removed at least partially, since proper crystallisation of the sugar is affected considerably by the degree of impurity of the raw juice. The conventional process for removal of non-sugar impurities is known as liming and carbonation and is based on calcium carbonate co-precipitation. The calcium carbonate is produced by adding lime and CO2 to the raw juice. The precipitated chalk and non-sugar impurities are filtering out, the calcium concentration is further reduced by decalcification using ion exchange technology. The next stage of the process is concentration of the juice in a multi-stage evaporator in order to raise the sugar content from about 10-16% by weight to about 60-70% by weight. For crystallisation, the syrup is further concentrated into a thick juice by boiling under conditions that allow for crystallisation. The resulting crystals are separated from the mother liquor by centrifugation, upon which the crystals are dried with hot air before being stored and/or packed.
As it is not feasible to crystallize all of the sucrose in the thick juice as commercially acceptable sugar product. A large amount of the sugar is lost to a discard called “molasses”. This inefficiency is largely due to the reality that the liming and carbonation “purification” procedures actually remove only a minor portion of the non-sucrose in the juice. The presence of residual non-sucrose in the thick juice significantly interferes with the efficient crystallization and recovery of the sucrose because of inherent crystallization and solubility effects.
Thus, the prior art methods for purifying sugar liquor and concentrating the sugar suffer from the fact that they are complex multi-step processes which consume large amounts of water and energy (approximately 15 cubic meters (m3) of water and 28 kilowatt-hours (kWh) of energy per metric ton of beet), limestone (approximately 3% on beet basis) and cokes (0.2% on beet basis). The methods produce substantial amounts of waste products (e.g. calcium carbonate precipitate or “mud”) and result in significant air emissions while resulting only in a limited purity of the thin juice and therefore require complex re-crystallization schemes. Altogether, the prior art methods are costly and inefficient.
U.S. Pat. No. 5,466,294 discloses an improvement of the process for purifying the raw juice obtained from sugar beets, outlined above. The process involves subjecting the raw juice to a chromatographic separation procedure utilizing an ion exchange resin. Although this process is based on ion exchange resins, the separation between sucrose and non-sucrose is based on ion exclusion rather than ion exchange. Ion exclusion is based on the fact that charged species (cations or anions) diffuse into the ion exchange matrix with more difficulty than small neutral molecules such as disaccharides or monosaccharides. The process disclosed in U.S. Pat. No. 5,466,294, however, has the disadvantage that the sugar juice is diluted and consequently large amounts of water have to be removed, which requires substantial amounts of energy, making it rather uneconomical. In addition it requires softening of the sugar juice.
U.S. Pat. No. 4,968,353 discloses another method for refining sugar liquor by the mineral cristobalite and an ion exchange resin. Cristobalite exhibits specific adsorbent properties for various colloidal or suspended substances, while the ion exchange resin exhibits decoloring and desalting properties with respect to colorants and salts. By combining refining by cristobalite and refining by the ion exchange resin, there is provided a sugar refining system. The process disclosed in U.S. Pat. No. 4,968,353 is based on ion exchange, which has a serious disadvantage that the process needs acids and bases to regenerate the ion exchange resins.